西澳大利亚纳沃日金矿流体混合与金的成矿作用

Fluid Mixing in the Generation of Nevoria Gold Mineralisation in the Southern Cross Greenstone Belt, Western Australia

流体包裹体研究表明，纳沃日金矿成矿脉体中流体包裹体类型有三种，富ＣＨ４、ＣＨ４－ＣＯ２－Ｈ２Ｏ和富Ｈ２Ｏ包裹体。初始成矿流体是富ＣＨ４热液，在主矿化阶段，另一来源的ＣＯ２－Ｈ２Ｏ流体与ＣＨ４流体发生了不均匀混合，并且在石英－磁黄铁矿阶段最为强烈，造成不同矿化部位的ｘＣＨ４变化极大，早期近于纯ＣＨ４流体被充分“稀释”后，石英－黄铁矿阶段的ｘＣＨ４显著减小，到成矿晚期只剩ＣＯ２－Ｈ２Ｏ流体。正是由于不同性质流体的混合，造成热液的ｐＨ及ｆｏ２的升高，流体中的金－硫络合物分解，金沉淀成矿。

The Nevoria deposit, a banded iron formation-hosted gold deposit, occurs within the roof of amphibolite-facies greenstones in the Southern Cross belt, Yilgarn Block, Western Australia. The mineralizing veins include early quartz-pyrrhotite±chalcopyrite veins and late quartz- pyrite veins. Three types of fluid inclusions, namely CH4-rich, CH4-CO2-H2O and H2O-rich ones, were found at Nevoria. Microthermometric data reveal that the initial auriferous fluids were CH4-dominant. Fluid mixing may have caused remarkable variations of CH4/CO2 ratios or xCH4 of CH4-CO2-H2O inclusions, particularly in mineralized quartz-pyrrhotite veins. The Caac- CQHZO inclusions in quartz-pyrite vein set have an apparent trend of decreasing xCH4 and increasing molar volume It can be inferred that, from the quartz-pyrrhotite stage to the quartz-pyrite stage, homogenization temperatures of CH4-CO2-H2O inclusions decreased, and CH4 contents (xCH4 ) also decreased. The carbonic phase in the inclu-sions varies from CH4-dominant to CO2-dominant. Phase mixing, resulting in an increase in pH and fo2, together with loss of reduced sulfur by mineral/fluid reactions and precipitation of sulfides, led to the breakdown of the gold-transporting complexes.